Induction plasma generator with gas sheath forming chamber



M. L. THORPE Dec. 8, 1970 INDUCTION PLASMA GENERATOR WITH (ms SHEATH FORMING CHAMBER Filed June 21, 1967 2 Sheets-Sheet 1 H6 FIG l Ell:

FIG 4 FIG 3 Dec. 8, 1970 Filed June '21, 1967 M. L. THORPE INDUCTION PLASMA GENERATOR WITH GAS SHEATH FORMING CHAMBER 2 Sheets-Sheet 2 I I I (\J/L o /L N N r m 7 I l O 0 o I. 9'. m

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. 8 a O V LO United States Patent US. Cl. 313231 8 Claims ABSTRACT OF THE DISCLOSURE A thermal plasma generator including a one inch I.D. quartz tube 3 inches long encompassed near one end by a water cooling jacket and near the other end by a water cooled pancake coil having a 1%; inch ID. A gas distributor having six radial passages is disposed in a housing structure which also receives in adjacent relation the jacket encompassed portion of the quartz tube. A gas sheath forming chamber having an annular width of inch and an axial length of two inches, is formed by the I.D.s of the gas distributor and the quartz tube and the OD. of an insert. Two parallel, spaced, flat plates, through which the quartz tube extends, are secured to and supported by the housing perpendicular to the axis of the tube and the water-cooled pancake coil is disposed between the plates and is supported by one of the plates in precise coaxial relation to the gas sheath forming chamber.

SUMMARY OF INVENTION This invention relates to induction thermal plasma generators.

Induction thermal plasma generators employ an intense electromagnetic field to create a thermal plasma. This plasma is useful for many purposes such as working metallic and refractory materials, in chemical reactions, and in other processes requiring high temperatures. For useful operation, the plasma established within the generator must be stable against drift and the heat reaching the walls of the plasma chamber must be dissipated with out damage to the integrity of that structure.

Heretofore, it has been diificult to reproducibly build reliable thermal plasma generators and it has generally been necessary to carefully adjust and control a large number of interrelationships of the generator in order to establish a stable plasma, such interrelationships including adjustment of the positions of several components and also the coordination of gas flow from several different gas sources.

Accordingly, it is an object of this invention to provide a novel and improved induction plasma generator which provides a stable plasma from a single gas source. Another object of the invention is to provide a novel and improved induction plasma generator which readily accommodates a variety of induction coil configurations and reproducibly generates a stable plasma condition.

Another object of this invention is to provide a compact induction plasma generator in which its several components may be'easily and quickly assembled into the desired precise interrelation so that the generator will reliably produce a useful thermal plasma.

Another object of this invention is to provide a generator which may be conveniently assembled in an accurate precise configuration and be disassembled and reassembled without the necessity for critical adjustment of components of the structure.

A further object of the invention is to provide a plasma generator of novel and improved configuration for increased versatility and practicality.

3,546,522 Patented Dec. 8, 1970 In accordance with the invention there is provided a plasma generator, the main components of which are dimensionally interrelated in a manner which results in a compact and yet versatile and easily operated plasma generator. The generator includes a chamber for forming a thin annular gas sheath which defines a main axis for the generator. The sheath forming chamber has a length at least five times the width of the sheath space beyond a gas distributor which at one end of the sheath chamber introduces gas uniformly around the annular chamber for flow through the chamber in the axial direction so that a thin sheath of gas of substantially uniform velocity around its entire circumference flows out of the sheath forming chamber. A tubular element defining a plasma forming chamber is disposed at the outlet of the sheath forming chamber with the outer wall of the sheath forming chamber aligned with the wall of the plasma forming chamber to produce a smooth flow of the sheath gas along the outer wall of the plasma forming chamber. An induction coil, which surrounds the plasma forming chamber, is located close to the sheath forming chamber and has an inner diameter comparable to the sheath diameter, the coil inner diameter being less than 1 /3 times the sheath diameter and preferably less than about 1 /5 times the sheath diameter to maximize the energy transference from the power supply to the plasma. This coil is supported in precise coaxial relation to the sheath at a distance less than three sheath diameters from the outlet of the sheath forming chamber by structure which mounts the coil and the plasma and sheath forming chambers together as a unit.

In particular embodiments, the plasma generator includes a housing structure having a main recess of substantial depth; a gas inlet port, a coolant inlet port and a coolant outlet port in communication with the main recess; and a second recess coaxially aligned with the main recess so that a shoulder defines the transition between the main and second recesses. An annular gas distributor structure which includes a plurality of discharge ports is seated in the main recess on the transition shoulder and has an annular channel in communication with the housing gas inlet port for distributing gas to the discharge ports. An annular coolant distributor structure which includes an annular channel in communication with the coolant inlet and outlet ports is also disposed in the main recess beyond the gas distributor structure. The tubular element defining the plasma forming chamber has an inner periphery of the same configuration as the inner periphery of the gas distributor and is disposed so that the inner peripheries of the gas distributor and the tubular element form a smooth continuous surface. The coolant distributor is in intimate contact with an intermediate length of the tubular element for removing heat therefrom. A disc structure secured to the end of the housing and overlying the main recess, clamps the tubular element and the gas and coolant distributors in fixed, predetermined position. An insert member, disposed in the second recess has a portion extending into the main recess to define the inner wall of the sheath forming annular chamber. The electrically conductive induction coil is secured to the disc clamping structure and surrounds the tubular element outside the housing for creating an intense, high frequency electromagnetic field in the plasma forming chamber for converting gas introduced by the gas distributor to plasma condition. A rigid cantilever support element, extending radially from the disc structure, supports the housing and tubular element in spaced relation from a support surface. In an embodiment employing a pancake coil, the disc structure includes two flat plates which are held in parallel relation by spacers, and the cantilever support element and disc structure form part of the electrical circuit of 3 the coil. Also in preferred embodiments the direction of the gas distributor discharge ports is radially inward.

The invention provides a compact plasma generator unit which produces a stable thermal plasma employing a single gas inlet. The major components of this plasma generator are of materials that may be fabricated easily to the precise dimensional relationships required in a relatively inexpensive manner. The generator accommodates dilferent coil configuration, including both pancake coil and helical coils.

Other objects, features and advantages of the invention will be seen as the following description of particular embodiments thereof progresses, in conjunction with the drawings, in which:

FIG. 1 is a perspective view of an induction plasma generator constructed in accordance with the invention;

FIG. 2 is a cross-sectional side view of the generator including all electrical and coolant connections in which the coolant inlet and outlet for the housing structure and a spacer have been moved into the plane of the view;

FIG. 3 is a cross-sectional view of the generator taken through the plane of the gas inlet along the line 33 of FIG. 2; and

FIG. 4 is a side view, partially in section of a second form of induction plasma generator constructed in accordance with the invention.

DESCRIPTION OF PARTICULAR EMBODIMENTS The plasma generator as shown in FIG. 1 includes a .rigid tubular projection for connection via coupling 12 to a suitable support member. Plate 14 is mounted at the end of tubular projection 10 and a second plate 16 is supported above plate 14 in parallel spaced relation by spacers 18. Mounted on and secured to plate 16 is a housing 20 in which is disposed one end of a quartz tube 22, 3% inches long and having a wall thickness of inch and an inner diameter of one inch which defines a plasma forming chamber. The gas to be converted to plasma condition is introduced through fitting 24 while a flow of cooling water is introduced to housing 20 via intake fitting 26 and exhausted through discharge fitting 28. A probe structure 30 for introducing material to be treated in the plasma formed in tube 22 has an enlarged head and extends through plug 32 into the housing 20 from the upper end thereof. A pancake coil 34 is disposed between plates 10 and 16 and surrounds tube 22. Coil 34, when energized, creates an intense electromagnetic field in tube 22 and increases the energy level of gas in that chamber to thermal plasma levels. Additional details of the plasma generator may be seen with reference to FIG. 2.

As shown in FIG. 2, rigid tubular projection 10 is connected by means of coupling 12 to a power supply structure 50. Coupling 12 is threadedly secured to a coupling terminal 52 which includes an electrical power supply terminal and a passage'for fluid coolant (such as water). The other end of coupling 12 is secured to round copper plate 14, 6-inches in diameter and inch thick, and having a radial slit (for preventing the flow of circulating current in the plate). An annular copper radiation shield 54, one inch long, is fitted over the outside of quartz tube 22 near the end of the tube projecting from housing 20. Copper plate 14 surrounds and is secured to the outside periphery of this radiation shield.

Round aluminum plate 16, six inches in diameter and inch thick, surrounds the outside periphery of quartz tube 22 at the point where the tube projects from the housing structure 20. Plate 16 is supported above plate 14 by three spacers 18, 1 /2 inches high, equidistantly located around the peripheries of the plates. The spacers 18 are secured to each of the plates by means of screws 56.

Plate 16 is secured by means of screws 58 to the surface of housing 20 through which tube 22 extends.

Rectangular aluminum housing structure 20 has a main recess 60 of two coaxial section The first section 62 is a 1 inch diameter counterbore; the second section 64 is also a counterbore, 1 inches in diameter, extending from the first section to the lower end of housing 20. A second recess 66 is a inch diameter cylindrical passage coaxial with the bores 62, 64 and extends from first counterbore section 62 at shoulder 68 to the upper end of housing 20.

First counterbore section 62 of main recess receives, in snug fitting relation, an annular gas distributor structure 70 which has an ID. of one inch. The inner end of the gas distributor structure is seated in the main recess 60 on shoulder 68, and the entire gas distributor extends the length of the first counterbore section 62.

An annular recess inch by inch in cross-section) on the outside of gas distributor 70 forms, with the housing, an annular gas supply channel 72 in communication with gas inlet 24 in housing 20. Six radial 0.028 inch diameter discharge ports 74 (see FIG. 3) extend from channel 72 to the inner wall of gas distributor 70. A second annular recess 76 on the outside of the gas distributor 70 above the gas supply channel 72 receives sealing O ring 78.

Second counterbore section 64 of main recess 60 receives, in snug fitting relation, an annular coolant distributor structure 80. Coolant distributor 80 has an annular groove 82, forming with the housing an annular fluid channel of depth equal to about half the thickness of the distributor, on its outer surface. Fluid channel 82 is in communication with fluid intake and discharge fittings 26, 28. Annular recesses 84, on either side of the fluid channel 82, receive 0 rings 86. The inner end of the coolant distributor structure is tapered and forms a recess for O ring 88. The coolant distributor structure 80 extends substantially the length of the second counterbore section 64.

Quartz tube 22 is fitted inside coolant distributor 80. The upper end of tube 22 extends past the end of coolant distributor 80 to be received by recessed seat at the outer end of gas distributor 70 adjacent its inner periphery. As the inner diameter of tube 22 is equal to the inner diameter of gas distributor 70, a smooth continuous surface is formed. The outer end of the coolant distributor abuts plate 16. Plate 16 thus clamps the gas and coolant distributors in the main recess 60 in fixed, predetermined position and forces 0 ring 88 inward to seal against tube 22 and distributor 70 so that tube 22 is firmly held in position abutting distributor 70.

An annular recess 92 on the inside surfaces of the second recess 66 of housing 20 receives an O ring 94 which provides a seal with respect to lipped, cylindrical base plug 32, M; inch in diameter and 2% inches long, which extends into the housing 20.Base plug 32 extends into the main recess 60 of housing 20 and has an outside diameter 4; inch smaller than the inside diameter of quartz tube 22, so that a thin annular gas sheath forming chamber 98 is formed between the outside wall of the base plug and the inside walls of the gas distributor 70 and the quartz tube 22. A cylindrical extension on plug 32 provides a two inch length for chamber 98 and a recess 99 which improves the stability of the generator operation.

Metal probe 30 is mounted in threaded fitting 100 which is threadedly secured to a threaded counterbore at the lipped end of base plug 32. Probe 30 extends through a central bore of the base plug and protrudes beyond the plug into the plasma chamber 102 defined by quartz tube 22. The probe is secured in sealing relation by an 0 ring 104 which acts against the tapered end surface of fitting 100 in cooperation with the bottom surface of the counterbore in the base plug 32. When the fitting 100 is loosened the probe 30 may be moved inwardly or outwardly to change the extent of its protrusion in plasma chamber 102.

The probe has a central passage surrounded by a double walled coolant channel which provides an ingoing annular path 106 connected to a threaded inlet fitting 108 in probe head structure 110 and an outgoing annular return path 112, surrounding the ingoing path, which is connected to threaded outlet fitting 114 in the probe head structure.

Material to be processed is injected into the plasma chamber 102 by introduction through inlet 116 at the upper end of the probe 30.

A spiral Wound copper tube covered with polytetrafluoroethylene insulation surrounds copper radiation shield 54 of tube 22 and forms a pancake coil 34 of 1 inch inner diameter which is secured to copper plate 14 between the copper plate and aluminum plate 16 and located concentrically ($0.002 inch) with respect to sheath forming chamber 98. The outside end of the copper tube extends to connector 120, which is coupled via coupling 122 to terminal 124, for an electrical connection to the high voltage terminal of the power supply 50. The inside end of the copper tube is electrically connected and mechanically secured to copper plate 14 by a connector block 126 to complete the coil circuit to terminal 52 (the ground terminal of the power supply). A coolant channel 128 extends through tubular projection 10, radially through copper plate 14 to a point below the connector block 126, and through that block to tube 34.

One end of a flexible fluid conduit 130 is attached intermediate the ends of tubular projection and the other end is connected to housing fluid coolant inlet fitting 26. One end of flexible fluid conduit 132 is connected to housing fluid coolant outlet fitting 28 and the other end is connected to probe head inlet fitting 108. One end of flexible fluid conduit 134 is connected to probe head outlet fitting 114 and the other end is connected to a fitting 136 which communicates with the fluid channel 138 of connector 120.

The inlet end of quartz tube 22 is cooled by fluid (such as water) supplied through conduit 130 to coolant dlS- tributor structure 80. The coolant fluid then flows through conduit 132 to the probe head structure 110. After passage through the probe cooling passages 106 and 112 the fluid continues through conduit 134 to connector 120. The pancake coil 34 and the outlet end of quartz tube 22 are cooled by flow of coolant fluid through the coil from tubular member 10 to connector 120.

In a typical operation, argon is introduced to the gas distributor channel 72 and flows radiall inwardly through ports '74 into the gas sheath forming chamber 98 between the gas injector structure 70 and the base plug 32. A thin annular sheath of gas is formed in space 98 and the gas flow retains this sheath form as it proceeds out of chamber 98 into the plasma chamber 102. In the plasma chamber the gas is immediately subjected to the electromagnetic field created when the pancake coil 34 is energized.

In startup, the four megacycle power supply is initially adjusted to a plate power of eight kilowatts. An arc is initiated by sparking which can be accomplished, fonexample, by the insertion of a grounded ignition rod into the chamber 102 or through the use of a Tesla coil. After the plasma 136 is created, a transition may be made from argon to a diatomic gas such as oxygen. During th1s transition appropriate adjustment is made in the operatlng plate power to maintain the gas in plasma condltion. In typical stable operation, an oxygen plasma has fifteen kilowatts of energy transferred to it at an oxygen flow rate of 58 s.c.f.h.

The arrangement and configuration of the elements of the plasma generator is such that it may be conveniently assembled in a simple yet accurate manner which does not require individual adjustment of the components. The gas distributor 70' is placed in snug fitting relation into the first section 62 of the housing main recess 60. Its seating on the transition shoulder 68 between the main recess 60 and the second recess 66 of the housing fixes its axial position. The coolant distributor 80- is similarly snugly fitted in the second section 64 of the housing main recess 60.

The quartz tube 22 is accurately positioned axially by being seated in the recessed seat in the inner periphery of the outer end of the gas distributor 70. The radial alignment of the tube is fixed by its snug fitting relation with both the gas distributors recessed seat 90 and inner wall of the coolant distributor 80 and its securing by O ring 88. This arrangement provides continuity between the inner surfaces of the quartz tube 22 and the gas distributor 70 which avoids turbulence in the gas sheath forming chamber 98. Plate 16 secures these components in place. The inside boundary of the gas chamber 98 is defined by the wall of base plug 32.

Tubular member 10 (a terminal of the pancake coil 34) functions as a rigid cantilever support element for the unit and is mechanically secured to plate 14, which supports plate 16 and housing 20 via spacers 18.

A second embodiment of the invention is illustrated in FIG. 4. The same housing structure 20 and quartz tube 22 configuration is utilized. A 1 inch I.D. helical coil is secured to clamping disc 16 by cantilever member 142 such that the coil is centered ($0.002 inch) in precise coaxial relation to plasma chamber 102 and sheath forming chamber 98, with the closest end of coil 140 within inch of the end of sheath forming chamber 98 and the remote end of coil 140 only two inches away. This compact generator configuration is easily mounted by means of the coil coupling 144 by which member 142 is secured to disc 16. The components may be readily disassembled and reassembled as no precise adjustment or particular orientation of components is required; and the generator operates, without the necessity of auxiliary stabilizing tech niques such as swirl gas ports, to produce a stable plasma with both monotomic and diatomic gases, in an arrangement which allows alternate use of pancake and helical coil configurations.

While particular embodiments of the invention have been shown and described, various modifications thereof will be apparent to those skilled in the art and therefore it is not intended that the invention be limited to the disclosed embodiments or to details thereof and departures may be made therefrom within the spirit and scope of the invention as defined in the claims.

What is claimed is:

1. A thermal plasma generator comprising a housing structure having a main reces of substantial depth, a gas inlet port, a coolant inlet port and a coolant outlet port in communication with said main recess, and a second re cess coaxially aligned with said main recess so that a shoulder defines the transition between said main and second recesses, an annular gas distributor structure seated in said main recess on said transition shoulder, said gas distributor structure including a plurality of discharge ports, an annular channel in communication with said housing gas inlet port for distributing gas to said dischange ports, and a recessed seat at its outer end adjacent its inner periphery, an annular coolant distributor structure seated in said main recess on said outer end of said gas distributor structure, said coolant distributor structure including an annular channel in communication with said coolant inlet and outlet ports, a tubular element of length substantially in excess of the depth of said main recess defining a plasma forming chamber, the inner periphery of said tubular element being of the same configuration as the inner periphery of said gas distributor structure and one end of said tubular element being seated on said recessed seat so that the inner peripheries of said gas distributor structure and said tubular element form a smooth continuous surface and said tubular element projects a substantial distance out of said housing structure, said coolant distributor structure being in intimate contact with an intermediate length of said tubular element for removing heat therefrom, a disc structure secured to the end of said housing structure and overlying said main recess, said disc structure clamping said gas and coolant distributor structures in said main recess in fixed, predetermined position relative to said tubular element, an insert member disposed in said second recess and having a portion extending into said main recess, said portion having the same crosssectional surface configuration as the inner periphery of said tubular element and being of slightly smaller dimension so that an annular chamber is defined having a length at least ten times its annular width, said distributor discharge ports comununicating with said annular chamber so that gas is introduced into said annular chamber for flow past said one of said tubular elements in a thin annular sheath of substantially uniform velocity about its circumference, a coaxial passage extending through said insert member for introducing material to be processed into said tubular element, an electrically conductive coil surrounding said tubular element outside said housing structure for creating an intense, high frequency electromagnetic field in said plasma forming chamber for converting gas introduced by said gas distributor structure to plasma. condition, said coil having an inner diameter less than 1% times the outer diameter of said annular chamber and being supported concentrically with respect to said annular chamber at a distance of less than three times the diameter of said annular chamber from the outlet of said annular chamber by said disc structure, and a rigid cantiliver support element extending radially from said disc structure for supporting said housing structure and said tubular element in spaced relation from a support surface.

2. The thermal plasma generator as claimed in claim 1 wherein said coil is a pancake coil.

3. The thermal plasma generator as claimed in claim 1 wherein said coil is a solenoid coil.

4. The thermal plasma generator as claimed in claim 3 wherein said disc structure included two flat parallel plates and spacers, said plates being held in spaced relation by said spacers.

5. The thermal plasma generator as claimed in claim 4 wherein said cantilever support element is integral with said disc structure and forms part of the electrical circuit of said coil.

6. The thermal plasma generator as claimed in claim 5 wherein said disc structure forms part of the electrical circuit of said coil.

7. The thermal plasma generator as claimed in claim 6 wherein said cantilever support element and said coil have coolant passages.

8. A thermal plasma generator comprising a housing structure having a main recess of substantial depth, a gas inlet port, a coolant inlet port and a coolant outlet port in communication with said main recess, and a second recess coaxially aligned with said main recess so thata shoulder defines the transition between said main and second recesses, an annular gas distributor structure seated in said main recess on said transition shoulder, said gas distributor structure including a plurality of discharge ports, an annular channel in communication with said housing gas inlet port for distributing gas to said discharge ports, and a recessed seat at its outer end adjacent its inner periphery, an annular coolant distributor structure seated in said main recess on said outer end of said gas distributor structure, said coolant distributor structure including an annular channel in communication with said coolant inlet and outlet ports, a tubular element of length substantially in excess of the depth of said main recess defining a plasma forming chamber, the inner periphery of said tubular element being of the same configuration as the inner periphery of said gas distributor structure and one end of said tubular element being seated on said recessed seat so that the inner peripheries of said gas distributor structure and said tubular element form a smooth continuous surface and said tubular element projects a substantial distance out of said housing structure, said coolant distributor structure being in intimate contact with the exterior surface of said tubular element along an axial length thereof'for removing'heat therefrom, struc ture for clamping said gas and coolant distributor structures in said main recess in fixed, predetermined position relative to said tubular element, an insert member disposed in said second recess and having a portion extending into said main recess, said portion having the same crosssectional surface configuration as the inner periphery of said tubular element and being of slightly smaller dimension so that an elongated annular chamber is defined, said ydistributor discharge ports communicating with said annular chamber so that gas is introduced into said annular chamber for axial flow past said one end of said tubular element in a thin annular sheath of substantially uniform velocity about its circumference, a coaxial passage extending through said insert member for introducing material to be processed into said tubular element, an electrically conductive coil surrounding said tubular element outside said housing structure for creating an intense, high frequency electromagnetic field in said plasma forming chamber for converting gas introduced by said References Cited UNITED STATES PATENTS 3,116,405 12/1963 Browning et al 219- 3,264,508 8/1966 Lai et-al '3l3-231 X- 3,324,334 6/1967 Reed 313-'23l 3,401,302 9/1968 JAMES w. LAWRENCE, Primary Examiner P. C. DEMEO, Assistant Examiner U.S. c1. X.R. 1 219-121; 313-161; 315- 1'11 Thorpe et al 315111 

